1. Field of the Invention
The present invention relates to an optical fiber communications network; and more particularly, to an optical fiber network system and method providing improved fiber utilization in the link between end-users and central stations.
2. Description of the Prior Art
Present telecommunications and computer systems require the high data-rate transmission of digital information between different circuits. These circuits may be in close proximity, such as within a single equipment cabinet, or they may be separated by very long distances. In the earliest stages, telecommunications involved transmission of electrical impulses carried using a wired connection, such as ordinary copper wires, a coaxial cable, or a conductive trace on a circuit board. Later, transmission was also carried wirelessly using microwave or satellite connections.
More recently, the alternative of transmitting data in the form of light pulses propagating through optical fibers has become increasingly prevalent, because of the vast increase in capacity afforded over what is possible either with either wired electrical connections or wireless satellite or microwave links. A single optical fiber, which may be thinner than a human hair, can carry far more data than a copper wire pair. In many circumstances, telecommunications providers already have physical right-of-way in the form of existing utility poles or underground conduit ducts. By replacing existing copper wires or cables in this right-of-way with optical fiber, demand for more bandwidth can be satisfied far more efficiently, and with less societal and environmental impact, than if new construction were required.
Optical fiber communication relies on the representation of binary digital data by a series of on/off light pulses. These pulses typically are generated by laser diodes (LDs) or light emitting diodes (LEDs) and injected into long fibers of glass or polymeric materials. The fibers are capable of propagating the light over extended distances with extremely low attenuation and dispersion, whereby information embodied in an on/off modulation pattern may be conveyed. The light pulses that emerge at the other end of the fiber can be detected and reconverted into electronic signals that reproduce the original electrical signal. Commonly, a single fiber is used for bidirectional communication, with the data transmitted in one direction represented by light pulses of one wavelength (color) and the data transmitted in the opposite direction represented by light pulses of a second wavelength. As used herein and in the subjoined claims, and in accordance with conventional parlance in the fiber optics art, the term “light” is employed for electromagnetic radiation that extends from the infrared to the ultraviolet, thus including both wavelengths perceptible to humans (about 380-750 nm) and wavelengths above and below the visible spectrum. For example, silica-fiber based systems frequently use wavelengths in the range of about 1.2-1.6 μm (1200-1600 nm), which are classified as near infra-red and are not visible to humans. Nevertheless, in the fiber-optic art, radiation at these wavelengths is still termed “light” and particular wavelengths are called “colors” by analogy.
Present-day optical fiber communication is often implemented in a telecommunications system in which a part of the network is generically called a passive optical network (PON) system. In the nomenclature of a typical telephony system, a central office or station may house some number of optical line termination stations (OLT) that provide an interface between electrical and optical signals. The OLTs are in communication on one side with data sources and on the other side are connected to an optical fiber that provides a bidirectional data path to end users. Each central office providing optical service must have at least one OLT, but usually there are a large number of OLTs.
At the other end, devices called optical network units (ONU) connect on one side to optical fiber from a central office OLT and on the other side through conductive wire to one or more customer devices, which can include telephones, computers, televisions, or the like. Normally, fiber from the central office is connected through a multiplexer/splitter to multiple ONUs. Each ONU is associated with a particular OLT, and at least one ONU is associated with each customer's location. The number of ONUs serviced by a given OLT depends on the amount of bandwidth each needs. The number of ONUs might typically be 32, but can also range up to about 256 or more in some circumstances.
The two directions of transmission are generally called “downstream” and “upstream,” and refer, respectively, to data going in the direction from the OLT to the ONU, and the reverse direction from the ONU to the OLT. The ONU includes the electronic devices needed to convert incoming optical signals to the electrical signals needed by the various customer devices. Likewise, the ONU receives electrical impulses from these devices and converts them to optical pulses for upstream transmission. Corresponding conversions between optical and electrical signals are performed by the OLT at the central office.
Various forms of PONs have evolved to meet the ever-increasing demand for higher bandwidth, i.e., the amount of information that can be communicated per unit time. In order to ensure continuing compatibility between systems, standardized protocols for PONs have been promulgated by governmental regulatory authorities and standards-setting bodies. Prominent standard-setting bodies include the Institute of Electrical and Electronics Engineers (IEEE) and the International Telecommunications Union (ITU).
One common high-speed PON data transfer protocol which can be implemented using the present system is the GPON protocol, which specifies bidirectional operation with a data rate of about 2.5 gigabits per second (Gbps) in continuous-mode (CNT) transmission in the downstream direction and 1.25 Gbps in burst-mode (BM) transmission in the upstream direction. Other PON protocols are also compatible with practice of the present invention.
A typical, generic fiber optic system of the prior art for telephony is shown in FIG. 1. A PON system, depicted generally at 10, includes a plurality of OLTs 12a-12d, each being associated with a group 14a-14d of one or more ONUs. For example, OLT 12a serves ONU group 14a. Each ONU, in turn, is associated with one or more user devices (not shown) for which it handles upstream and downstream data transmission. It will be understood that while FIG. 1 illustratively shows a PON with four OLTs, the actual number may vary, typically from 1 to 8 or more. Likewise, each OLT may serve a number of ONUs ranging typically from 8 to 128 or more. Optical fibers 16, typically made of silica, connect each pair of nodes M and N, which may be separated by distances of up to about 20 km or more. Most commonly, the OLTs 12 and nodes M are all located in a single central office 8 of the telecommunications provider. At the user end, such as in an office building housing one or more office tenants, nodes N might all be located in one or more interior equipment cabinets or closets, with branches running to the various users. Individual nodes N might also be located in an exterior cabinet, which might be mounted on a utility pole, a ground-level pad, or in an underground vault, to serve end users in one or more buildings.
Signals from the OLT to the ONU in system 10 are carried through a fiber 16 as light pulses of a downstream wavelength, e.g. 1490 nm, while signals from the ONU to the OLT (upload or upstream direction), also carried through fiber 16, are assigned a different upstream wavelength, e.g. 1310 nm. The OLTs and ONUs provide an interface between optical fiber and electrical signals. That is to say, they convert between the upstream and downstream optical signals and corresponding electrical signals needed to connect with devices such as computers, telephone instruments, televisions, and other such implements. Each node N includes a splitter, which connects the incoming fiber to a plurality of fibers extending to the ONUs of that node's group. The splitter divides the optical intensity in the downstream data among the various ONUs, so each receives all the data. Conversely, the splitter aggregates (or multiplexes) upstream traffic from the various ONUs and injects the aggregated optical signal into the fiber serving the node for upload to the specified OLT. A suitable networking protocol implemented using a media access control (MAC) system is employed to identify and maintain the integrity of both the upstream and downstream data associated with each OLT, ONU, and end-user devices, and to govern the requisite routing and processing of the data particular to each end user. Frequently, the identity and integrity of the data in such a system is established by including in the data being exchanged suitable headers, addressing information, and delimiters and providing control signals that govern the timing of data transmission by the various devices and sources. MAC systems having the requisite capability for carrying out these functions are conventionally used in the telecommunications art.
The typical distance of up to about 20-30 km between nodes M and N in FIG. 1 arises from an interplay between the available optical power of feasible light sources, the attenuation of optical signals propagating through typical fibers, the amount of optical power available in each channel after division by the splitter, and the electronic sensitivity of typical optical receivers.
FIG. 2 depicts a graph showing the loss characteristic (dB/km loss) of conventional, single-mode silica fiber versus wavelength in the range of interest. It can be seen that some wavelengths are substantially more strongly attenuated than others. Various phenomena are believed to contribute to the losses, including Rayleigh scattering, which dominates at low wavelengths, and infrared absorption 22 by the fiber itself, which dominates at high wavelengths. Both these loss mechanisms vary relatively smoothly with wavelength. In addition, localized absorption in the fiber, caused by various impurities, including metals (peaks 24) and hydroxyl ions (peaks 26), produces absorption peaks centered over certain characteristic and relatively narrow wavelength ranges. These contributions together result in wavelength-dependent loss characteristic 28. Although fiber manufacturers have worked assiduously to make purer, more uniform fibers that somewhat reduce the absorption both in the impurity bands and overall, they cost much more. Moreover, it would be expensive and difficult to replace the vast amounts of older-generation fiber already in service, so that systems compatible with the installed base are particularly sought.
The widely used 1490 and 1310 nm base wavelengths are chosen because they are at approximate local minima in the absorption characteristic curve, while the 1550 nm wavelength is kept for optical video service. Light of the 1490, 1310, and 1550 nm wavelengths can transit 20-30 km of fiber without excessive attenuation or dispersion.
As a result of the continually increasing demand for high bandwidth digital data transmission, existing fiber installations are beginning to lack sufficient capacity to carry the desired amount of information. The problem is particularly acute in metropolitan areas, where installing new lines is especially difficult and expensive. Techniques that would increase the available bandwidth of existing fiber links are highly sought, in order to forestall or eliminate the need to install and maintain additional fiber connections. Especially desired are systems in which a single fiber could be used to connect multiple OLTs with multiple ONUs.